1. Field of the Invention
The present invention relates to a discharge lamp operating apparatus capable of reducing a curvature of a discharge arc due to gravity-induced convection caused when a discharge lamp, particularly a HID lamp (high intensity discharge lamp) is horizontally lighted.
2. Related Art of the Invention
In recent years, HID lamps have been widely used in outdoor lighting field and the like because of features of high efficiency and long life. Of these, metal halide lamps have good color rendering property, and are becoming widespread in not only the outdoor lighting field, but also the indoor lighting field by making the best use of the special property, and further are receiving attention as a light source for video equipment and as a light source for head lamps of vehicles.
In a conventional discharge lamp operating apparatus, when operating is made with a several KHz or more high frequency, a harmful acoustic resonance phenomenon occurs, and variations, flame failure, and the like of the discharge arc have occurred. As a method for solving such problems, there is a method described in Collection of Theses No. 10 of Tokyo Branch Conference of the Lighting Society in 1983. It is disclosed that this discharge lamp operating apparatus is capable of preventing variations, flame failure and lamp destruction of the discharge arc resulting from an acoustic resonance phenomenon peculiar to a HID lamp, which is a discharge lamp, and stably operating by supplying rectangular wave current of a low frequency (several hundred Hz) to the HID lamp.
When, however, the HID lamp is horizontally lighted in a operating apparatus of this type, operating can be stably made, but the discharge arc upwardly curves as a bow is drawn under the influence of gravity-induced convection caused due to temperature distribution occurring within the discharge space. This greatly increases the temperature in the upper portion of the discharge space to cause significant degradation, i.e., devitrification or deformation to quartz glass, which is glass envelope material for defining the discharge space, thus leading to a problem that the lamp life will become shorter, and the lamp efficiency will be lowered due to the lowered coldest point temperature in the lower portion of the discharge space.
Particularly in recent years, in the metal halide lamp, which is receiving attention as a light source for video equipment and a light source for head lamps of vehicles, the use of shorter arcs is proceeding, and it is necessary to further increase the pressure which is caused by the filling material in the discharge space while the lamp is operating. The increase in the pressure increases the gravity-induced convection, making the curvature of the discharge arc larger, and further worsening the lamp life and the efficiency.
As a discharge lamp operating apparatus which solves this problem, there is one described in Japanese Patent Publication No. 7-9835. The discharge lamp operating apparatus will be described with reference to FIG. 10.
FIG. 10 is a view showing variations in lamp current waveform with respect to lapsed time after operating when a discharge lamp is caused to light in a conventional discharge lamp operating apparatus (Japanese Patent Publication No. 7-9835) described above. This is obtained by superposing AC waveform 52 on DC waveform 51. The operation of this AC waveform 52 causes acoustic resonance of a specified pattern to reduce the influence of gravity-induced convection of the filling material in the discharge lamp, and as a result, the discharge arc becomes straight.
As this physical background, it is considered that a compression wave of a frequency determined by the AC waveform 52 is produced from the discharge arc to generate a standing wave, which forcibly causes the discharge arc, which is going to curve, to become straight.
When the discharge arc is made straight, there are advantages that it is possible to lower the temperature of quartz glass in the upper portion of the discharge space, which is the cause for shortening the life of the discharge lamp, for extending the life of the discharge lamp, and to improve the luminous efficiency by an increase in the coldest point temperature in the lower portion of the discharge space among others.
Further in FIG. 10, if the level of the DC waveform 51 is made high in advance in a period until the discharge lamp reaches operation under the rated condition, it will be possible to obtain a predetermined optical output immediately after operating. At the same time, it will be possible to reduce a rate (a value obtained by dividing what obtained by subtracting the minimum value from the maximum value of the AC waveform 52, by the value of the DC waveform 51) of a ripple occurring in the AC waveform 52 to the level of the DC waveform 51 during the period until the discharge lamp reaches operation under the rated condition. Therefore, it suggests that the discharge arc does not become unstable, but it is possible to obtain a stable discharge arc from the beginning of operating of the discharge lamp even if gas temperature within the discharge space changes, and the frequency which makes the discharge arc straight changes during the period until the discharge lamp reaches operation under the rated condition.
In the above-described conventional discharge lamp operating apparatus, however, since an electric current flows through the discharge lamp in one direction, there always arises an electric field in one direction although the electric field intensity in the discharge space of the discharge lamp changes periodically. For this reason, there was a problem that irregular color occurs in the discharge arc because of a cataphoresis phenomenon in which filling material shifts toward one side. Also, since temperature on the electrode shaft is in an asymmetrical temperature distribution in which the temperature on the anode side is high while that on the cathode side is low, the curvature of the discharge arc becomes more or less smaller, but does not become that much straight even if a frequency which excites a mode in which the discharge arc is made straight is superimposed.
The present inventor et al. have referred to Japanese patent application (Japanese Patent Application No. 8-220938, 9-69502 and 9-225001) or U.S. patent application Ser. No. 08/915641).
The content is as follows: A discharge lamp operating apparatus for operating a discharge lamp, having a glass envelope for defining a discharge space and a pair of electrodes which are hermetically filled within the glass envelope and are opposed to each other, comprising:
generation means for generating a waveform signal having a frequency component of an acoustic resonant frequency for exciting a mode in which a discharge arc occurring between the pair of electrodes is made straight, and whose waveform center line is at a constant level; and PA1 modulation means for modulating the waveform signal so that the polarity is alternately changed at a modulation frequency which
has the center line of the waveform signal lower than the frequency component of the acoustic resonant frequency".
In accordance with the invention of this prior application, a sufficiently straight discharge arc could be realized. Even in the invention of the prior application, the following can be considered:
That is, in the case of operating after the discharge lamp has sufficiently long extinction, the discharge space has uniform temperature distribution which substantially coincides with the ambient temperature of the discharge lamp, and therefore, the discharge is not greatly deformed, but is most easily performed in the shortest distance between a pair of electrodes, that is, a straight track at dielectric breakdown when discharge starts, and thereafter, this discharge can develop into a discharge arc to obtain a stable, straight discharge arc.
In contrast, in the case of operating again before the extinction is sufficiently long, there can be expected a drawback that there may be cases where no stable, straight discharge arc could be obtained.
This is attributed to the fact that under the influence of temperature distribution (position where the discharge arc exists: about 5,000 K to 7,000 K, near the glass envelop: about 1,000 K) in the discharge space which occurs during operating of the discharge lamp, the temperature in the vicinity of the center of the discharge space where the discharge arc existed is higher than that in the vicinity of the glass envelop for a predetermined period even after the light is put out.
When discharge from the discharge lamp starts in a state having un-uniform temperature distribution within the discharge space, the high-temperature portion in which a discharge arc existed has significant kinetic energy of particles within the discharge space, is greatly curved so as to avoid it for discharging, and since this discharge curved becomes a starting point, a distorted discharge arc is formed between a pairs of electrodes.
Once a discharge arc, which is a source for generating a compression wave, is curved and generated, the compression wave is generated from the curved state and becomes stable in a curved state as it is.